This study will compare the effects of a 3-month, individually prescribed progressive exercise training program on: 1) chronic pain (CP), depression and PTSD symptoms, and 2) neurobiological and related neuropsychological mechanisms by which our exercise-training paradigm may foster exercise maintenance. More specifically, we will investigate hypothesized relationships between exercise-training associated augmentation of neuropeptide Y (NPY) system function and improved capacities for reward and self- regulation?neuropsychological capacities posited to underlie intrinsic motivation and self-efficacy, which in turn have been shown to predict exercise maintenance. This study will focus on Veterans and non-veterans with CP/PTSD. The study design includes a baseline, acute, cardiopulmonary exercise assessment (CPX) that will inform the exercise prescription for the 12-week ?progressive exercise? training program, comprised of three 30-45 minute exercise sessions per week (walking, running, cycling or swimming, depending on the ability/capacity of the participant). Exercise sessions will be initially supervised by an exercise physiologist in the Clinical Studies Unit (CSU) at VA Boston Healthcare System, and then transition into the home. Intermittent telephone calls by the researchers will provide additional motivational support and problem solving. Implementation of the prescribed exercise regimen will also be supported by the use of heart rate and actigraph monitors programmed for the participant to achieve their prescribed heart rate range (HRR). Finally, an ?endpoint? CPX assessment will track changes in NPY system function and delineate their impact on pain, depression and PTSD symptoms, as well as the factors proposed to foster exercise maintenance. Both CPX tests will be performed in accordance with guidelines published by the American College of Cardiology. Among Veterans and non-veterans with CP/PTSD, we hypothesize that the capacity to release NPY in response to vigorous exercise (i.e., acute CPX testing) will be associated with improvements in pain, depression and PTSD symptoms, as well as the putative factors that predict exercise maintenance. To have sufficient power to test these hypotheses, 30 participants (15/year), will be engaged in exercise training. Data from this R21 will be used to demonstrate feasibility and inform the further development of individually prescribed, motivationally based exercise regimens that could be used as adjuncts to cognitive and other therapeutic PTSD, depression or chronic pain interventions to reduce chronic musculoskeletal pain, depression and PTSD, as well as the negative consequences of these disorders over the long-term.